1. Field of the Invention
The present invention relates to a spark plug for use in an internal combustion engine and to a method for manufacturing the same.
2. Description of the Related Art
A typical spark plug used for ignition in a gasoline internal combustion engine for automobile use, includes a center electrode, an insulator disposed around the center electrode, a metallic shell disposed around the insulator, and a ground electrode joined to the metallic shell so as to form a spark discharge gap in cooperation with the center electrode. The spark plug is mounted on the cylinder head of the engine by means of threads formed on the outer circumferential surface of the metallic shell. Generally, the metallic shell is made of an iron-based material, such as carbon steel, and in many cases is plated with zinc for corrosion protection. Further, in order to improve anticorrosive performance, the surface of the zinc plating layer is subjected to chromate treatment.
For plating the metallic shell with zinc, so-called barrel plating is effective in terms of productivity. However, if the ground electrode is joined to the metallic shell through, for example, resistance welding after the barrel plating process is performed, a poor weld is likely to result, due to the presence of the zinc plating layer (or chromate layer). Also, since the zinc plating layer is destroyed at the weld zone, corrosion resistance is impaired. Generally, therefore, the ground electrode is joined to the metallic shell, and subsequently the resultant metallic shell assembly is plated with zinc through barrel plating and, as needed, subjected to subsequent chromate treatment.
Manufactured in accordance with the above-mentioned method, the surface of the ground electrode welded to the metallic shell is also covered with the zinc plating layer and in some cases with an additional chromate layer. If the insulating chromate layer covers a portion of the ground electrode which is to define a spark gap, discharge performance will be adversely effected. In order to improve resistance to spark-induced consumption, it has been determined that a chip containing a predominant amount of a high melting point metal, such as Pt or Ir, is welded to both the ground electrode and the center electrode at respective positions corresponding to the spark gap so as to form spark portions. However, when the chip of a high melting point metal is welded to the ground electrode, the existing zinc plating layer, and in some cases additional chromate layer, may cause a poor weld, potentially causing separation of the chip from the ground electrode.
In order to avoid the above-mentioned problem, a base metal of a free-end portion of the ground electrode is exposed without being covered with the zinc plating layer, so as to weld the chip of a high melting point metal to the exposed base metal. Conventionally, in order to maintain a free-end portion of the ground electrode in a bare state, a rubber tube, for example, is fitted to the free-end portion so as to prevent contact with a plating solution, thereby preventing formation of the zinc plating layer thereon. Because of difficulty in fitting the rubber tube to the free-end portion and difficulty in automating the fitting process, this method is of very poor efficiency. Also, when the metallic shell assembly is handled for fitting the rubber tube to a free-end portion of the ground electrode, oil and dirt are likely to adhere to the free-end portion. Such adhering oil and dirt impair welding of the chip, potentially causing separation of the chip from the ground electrode.
The present invention provides a method for manufacturing a spark plug including a center electrode; an insulator disposed around the center electrode; a cylindrical metallic shell disposed around the insulator; a ground electrode disposed in opposition to the center electrode so as to form a spark discharge gap therebetween; and a chip of a high melting point metal welded to at least the ground electrode at a position corresponding to the spark discharge gap so as to form a noble metal spark portion having a discharge face. The method comprises the steps of preparing a metallic shell assembly through joining of a root-end portion of the ground electrode to an open end portion of the metallic shell; forming a zinc-based plating layer containing a predominant amount of zinc on a surface of the metallic shell assembly; removing the zinc-based plating layer from a free-end portion of the ground electrode of the metallic shell assembly covered with the zinc-based plating layer; and welding the chip of a high melting point metal to the free-end portion of the ground electrode from which the zinc-based plating layer has been removed.
As described above, the method of the present invention does not involve a conventional masking process; i.e., a step of masking the free-end portion of the ground electrode with a rubber tube in order to prevent formation of a plating layer thereon. Thus, the invented method can manufacture at high efficiency a spark plug having a chip of a high melting point metal, serving as a spark portion, welded to the ground electrode. Also, the invented method avoids adhesion of dirt and oil which would otherwise result from employment of the masking process. Through execution of the step of removing the zinc-based plating layer, the surface of the base metal of the ground electrode is appropriately eroded to thereby be cleaned. As a result, weld strength of the chip is enhanced, thereby avoiding separation of the chip from the ground electrode.
A physical process, such as ion etching or shotblasting, may be employed in the step of removing the zinc-based plating layer. However, preferably, the ground electrode covered with the zinc-based plating layer is immersed in a remover so as to chemically remove the zinc-based plating layer therefrom. This chemical process enhances the efficiency of the removal. Also, the etching effect of the remover brings about cleaning of the surface of the base metal of the ground electrode.
Preferably, in order to chemically remove the zinc-based plating layer from the ground electrode covered with the zinc-based plating layer, the covered ground electrode is immersed in an acid remover so as to electrolessly remove the zinc-based plating layer therefrom. Preferably, the acid remover contains at least any one of nitric acid, hydrochloric acid, sulfuric acid, and an organic acid. This acid remover exhibits excellent performance in removing the zinc-based plating layer and thus can be used effectively in the present invention. Particularly, a mixture of nitric acid and hydrochloric acid is preferred as the acid remover. This mixture not only exhibits excellent performance in removing the zinc-based plating layer, but also is less likely to impair or discolor the surface of the base metal of the ground electrode, when the base material is an Ni-based metal (for example, an Ni-based heat resistant alloy) or an Fe-based heat resistant alloy. The base metal exposed through removal of the zinc-based plating layer exhibits good weldability to the chip of a high melting point metal. Preferably, in order to remove the zinc-based plating layer from a free-end portion of the ground electrode, the ground electrode is immersed in the remover such that a predetermined length of the root-end portion is exposed above the surface of the remover while the remaining free-end portion is submerged in the remover, thereby removing the zinc-based plating layer from the submerged free-end portion. Through mere adjustment of the depth of immersion of the ground electrode in the remover, the zinc-based plating layer can be removed from a predetermined length of the free-end portion.
Preferably, when the spark plug is configured such that a free-end portion of the ground electrode is bent so as to form the spark discharge gap in cooperation with an end portion of the center electrode, the zinc-based plating layer is removed from the free-end portion of the ground electrode in the following manner. The metallic shell assembly subjected to the process for removing the zinc-based plating layer is configured such that the ground electrode before being bent is joined to the metallic shell in such a manner as to extend linearly in the axial direction of the metallic shell assembly. The metallic shell assembly is held such that the ground electrode extends downward so as to immerse the free-end portion of the ground electrode in the remover. This method enables simple removal of the zinc-based plating layer from the free-end portion over a constant length, through a simple operation of holding the metallic shell assembly at a predetermined height by use of a jig or the like, while maintaining the level of the remover constant.
Preferably, the chip of a high melting point metal contains a predominant amount of Pt so as to facilitate resistance welding thereof. The surface of the base metalxe2x80x94which has been exposed through removal of the zinc-based plating layer and to which the chip is weldedxe2x80x94is activated through use of the acid remover, thereby promoting mutual diffusion between the chip material and the ground electrode material in the course of resistance welding. Therefore, the resultant spark portion of the ground electrode assumes high weld strength. This effect becomes remarkable when the ground electrode is formed of an Ni-based metal (for example, an Ni-based heat resistant alloy) or an Fe-based heat resistant alloy.
When the ground electrode is formed of an Ni-based metal containing a predominant amount of Ni or an Fe-based metal containing a predominant amount of Fe, the above-described method realizes the following spark plug according to the present invention. That is, there can be provided a spark plug which comprises a center electrode; an insulator disposed around the center electrode; a metallic shell disposed around the insulator; a ground electrode disposed in opposition to the center electrode so as to form a spark discharge gap therebetween, the ground electrode being formed of an Ni-based metal containing a predominant amount of Ni or an Fe-based metal containing a predominant amount of Fe; and a Pt-based metal chip containing a predominant amount of Pt and welded to the ground electrode, wherein a surface of said metallic shell and a surface of a root-end portion of said ground electrode are covered with a zinc-chromate layer including a zinc-based plating layer containing a predominant amount of zinc and a chromate layer covering the zinc-based plating layer, such that a free-end portion of the ground electrode is exposed; the Pt-based metal chip is welded to the exposed free-end portion of the ground electrode at a position corresponding to the spark discharge gap so as to form a noble metal spark portion; and a diffusion layer, formed at an interface where the noble metal spark portion and the ground electrode are joined, has a thickness of not less than 10 xcexcm.
Since mutual diffusion between the chip material and the ground electrode material is promoted in the course of resistance welding, the diffusion layer formed at the interface where the noble metal spark portion and the ground electrode are joined reliably attains an average thickness of not less than 10 xcexcm, thereby enhancing weld strength of the spark portion. As a result, separation of the chip is less likely to occur, even during high load operation.
Preferably, in order to enhance corrosion resistance of the metallic shell, a chromate layer is formed on the zinc-based plating layer through chromate treatment. This chromate treatment can be performed irrespective of whether or not the spark portion made of a high melting point metal is formed on the ground electrode. The present invention provides a method for manufacturing a spark plug which involves this chromate treatment. Specifically, the present invention provides a method for manufacturing a spark plug comprising a center electrode, an insulator disposed around the center electrode, a metallic shell disposed around the insulator, and a ground electrode disposed in opposition to the center electrode so as to form a spark discharge gap therebetween. The method comprises the steps of: preparing a metallic shell assembly through joining of a root-end portion of the ground electrode to an open end portion of the metallic shell assuming the form of a cylinder; forming a zinc-based plating layer containing a predominant amount of zinc on the surface of the metallic shell assembly excluding a free-end portion of the ground electrode, such that the free-end portion of the ground electrode is exposed without being covered with the zinc-based plating layer; and immersing in a chromate treatment liquid the entire metallic shell assembly which has undergone the step of forming the zinc-based plating layer, thereby subjecting the zinc-based plating layer to chromate treatment.
A spark plug according to the present invention is manufactured by the above-described method. The spark plug comprises a center electrode; an insulator disposed around the center electrode; a metallic shell disposed around the insulator; and a ground electrode disposed in opposition to the center electrode so as to form a spark discharge gap therebetween. The surface of the metallic shell and the surface of a root-end portion of the ground electrode are covered with a zinc-chromate layer including a zinc-based plating layer containing a predominant amount of zinc and a chromate layer covering the zinc-based plating layer, such that a free-end portion of the ground electrode is exposed without being covered with the zinc-based plating layer. In the zinc-chromate layer, the chromate layer is formed in such a manner as to cover an axial end face of the zinc-based plating layer with respect to the axial direction of the ground electrode.
Preferably, the chromate treatment is performed after completion of the step of removing the zinc-based plating layer from the free-end portion of the ground electrode. If, after chromate treatment is completed, chemical removal of the zinc-based plating layer through immersion of the ground electrode in a remover is performed, the formed chromate layer serves as a protection film, causing difficulty in removing the zinc-based plating layer. Also, at a position where the zinc-based plating layer and the chromate layer terminate to thereby cause exposure of the surface of a base metal, an end face of the zinc-based plating layer is exposed without being covered with the chromate layer. As a result, the zinc-based plating layer is likely to be corroded from the end face thereof. However, if the chromate treatment is performed after the zinc-based plating layer is removed from the free-end portion of the ground electrode, the chromate layer covers the remaining zinc-based plating layer including the end face thereof, thereby further enhancing corrosion resistance.
Thus it is an object of the present invention is to provide a method for manufacturing a spark plug capable of efficiently forming a zinc plating layer on an integrated assembly of a metallic shell and a ground electrode excluding a free-end portion of the ground electrode.
It is a further object of the present invention is to provide a spark plug manufactured by the method, particularly a spark plug which is less likely to involve poor welding of a chip of a high melting point metal to the free-end portion to thereby avoid the potentiality of separation of the chip from the ground electrode.